For reasons of driving safety, it is desirable to undertake the electric control of a vehicle damper in such a manner that the driving safety remains ensured even when there is a failure of the damper control because of a defect in the control unit or in the case of a fault with the current supply. This greater driving safety is more likely provided with a harder damping than with a softer damping. For this reason, the control should basically be so designed that the harder damping characteristic becomes effective when there is a fault of the control; whereas, the damping by means of the electric control reacts softer and therefore with greater comfort.
It is known to control semiactive dampers in accordance with the skyhook method. In the skyhook method, a desired damping force is computed which is proportional to the body velocity. In electrically adjustable dampers, a desired current is computed (in general, with an inverse damping characteristic field) from the desired damping force and the determined damping velocity and, with this desired current, an electrical valve changes the damping force in the damper. Because of the characteristic of a passive damper, only the least damaging damping force (the smallest possible damping) is adjusted when there are mutually opposite signs of damping velocity and required damping force.
The parameters of the skyhook controller (especially the proportional coefficient with which the vehicle body velocity is evaluated) or, more generally, the parameters of a characteristic field with which the body velocity is evaluated, are optimized usually with respect to the following: a low body excitation, low wheel load fluctuations and additional subjective criteria such as vehicle handling based on the evaluation of many vehicle maneuvers and these parameters are fixedly adjusted. For a fixed adjustment, a compromise must always be made between the driving comfort and the vehicle handling characteristics. A significant quality advantage is obtained when the control system recognizes specific driving maneuvers wherein the basic matching of the dampers is not used but (adapted in correspondence to the specific driving maneuver) the optimal damping is adjusted in a targeted manner. In the literature, this is done, for example, by changing the proportional coefficient of the body velocity.
The skyhook controller has the disadvantage that the damping force is continuously switched between hard and soft in dependence upon the damper velocity. This can, for example, lead to unwanted acoustic problems. A further disadvantage of the skyhook controller is that the damper automatically adjusts to “soft” when the body velocity is not present. This can lead to unwanted velocity movements (for example, at high vehicle speeds or braking and/or steering maneuvers) and therefore to a deterioration of the driving comfort.
The skyhook method is disclosed, for example, in U.S. Pat. No. 5,172,929 and German patent publication 4,115,061.